Green roofs, with special evaporation and good insulation performance, have the same capacity as passive cooling device of architecture. In this paper, the role of soil moisture in the insulation system of green roof was studied by numerical simulation. A comparison of influences of different moisture on roof heat transferring under three different modes of irrigation-intermittent irrigation, continuous irrigation and no irrigation was conducted. It’s concluded that roof heat flux was negatively related to soil moisture, and the heat flux of roof was more sensitive to soil moisture in intermittent irrigation mode. When the content of soil moisture was less than 0.2, the heat flux decreased significantly while the humidity was increasing. When the moisture content was more than 0.3, the heat flux had a relatively slow reaction. It was in continuous irrigation mode that the heat insulation performance of roof improved 3-4 times than the performance in intermittent irrigation.